Hello,
Melanomas are usually >6 mm in size,with irregular borders ,asymmetrical and dark in color. Melanoma has an irregular border, is asymmetrical,uneven color and increasing in size. If you notice changes in the mole's/mark’s color, height, size or shape, you should have a dermatologist evaluate it. Regarding the appearance of the layer of the lesion that is removed by biopsy, According to Clark's level (involvement of skin structures) and Breslow's depth (measured in millimeters) and published in Wikipedia, the preferred surgical margin for the initial biopsy should be narrow (1 mm) in order to prevent the disruption of the local lymphatic drainage and malignant cells if any can be seen in this biopsy after staining under a microscope.
I hope it helps. Take care and please do keep me posted in case you have any additional doubts. Kind regards.